This invention relates to a process and apparatus for integrating an alkene derivative process, such as an ethylene derivative process, with an ethylene process.
Many well-known processes are used for the manufacturing of alkene derivatives such as ethylene oxide and vinyl acetate monomers. Generally, these processes employ a purge to prevent build up of inerts in the alkene derivative production system. This purge stream can contain ethylene concentrations as high as 30 mole percent in an ethylene oxide process and as high as 80 mole percent in a vinyl acetate process. Depending on the process used, the amount of ethylene lost in the purge could be 1% or more of the amount required for the derivative process. The prior art has generally focused on recovering ethylene from the purge stream using separation schemes employing absorption, adsorption or membrane systems and could be located near the alkene derivative reactor of the alkene derivative process. For example, ethylene, oxygen and other reactants may react to produce an alkene derivative from a reactor. Process gas containing unconverted reactants in process gas treatment unit may be treated to remove carbon dioxide and then recycled back to the reactor. A purge gas stream from the process gas treatment may be vented to prevent build-up of inert or undesirable components in the process loop. While some plants incinerate the purge stream, others recover valuable ethylene from the purge stream for recycle to the reactor. An ethylene recovery unit may be used for this purpose, producing ethylene rich stream for recycle to the reactor and ethylene depleted stream, which is vented to reject the inerts. In some cases, ethylene depleted stream may be incinerated or used as a fuel. The ethylene recovery unit may be an absorption means, membrane means, pressure swing adsorption (PSA) means alone or in combination with a cryogenic unit.
U.S. Pat. No. 4,769,045 discloses a process for the direct oxidation of ethylene to ethylene oxide, in which ethylene is recovered from normally vented gas by contacting first with an activated carbon adsorbents and then by pressure swing adsorption with a zeolitic molecular sieve adsorbent.
U.S. Pat. No. 5,519,152 discloses a process of manufacturing ethylene oxide. Ethylene and free oxygen are reacted in the presence of methane, carbon dioxide, and argon in a reactor to form an effluent comprising ethylene oxide. The effluent is then withdrawn and ethylene oxide is removed from the effluent to obtain a recycle stream. The carbon dioxide and argon are removed from the recycle stream to obtain a treated recycle stream, which is supplied to the reactor along with additional reactants and methane.
U.S. Pat. No. 5,518,527 discloses a method for recovering ethylene from ethylene-containing vent gas from a plant for production of ethylene oxide, wherein ethylene is separated from saturated hydrocarbons such as methane, ethane and oxygen and then recovered efficiently. In this method, the vent gas contacts with molecular sieve carbon to selectively adsorb ethylene without substantial adsorption of the oxygen contained in the vent gas. The ethylene is then desorbed and recovered.
U.S. Pat. No. 4,904,807 discloses a process for producing ethylene oxide, which minimizes unreacted ethylene losses through the use of semi-permeable membrane units, thereby allowing an effective, selective removal of argon from the process cycle gas, without significant ethylene losses.
U.S. Pat. No. 4,879,396 discloses a process for producing ethylene oxide utilizing suitable semipermeable membrane units to selectively remove desired amounts of both carbon dioxide and argon diluents from the reaction recycle gas. This process makes available a much cheaper low purity oxygen source notwithstanding feed purge loss.
U.S. Pat. No. 5,817,841 discloses a process and apparatus for ethylene oxide production. A membrane unit containing a membrane selectively permeable to ethylene over argon is used to recover ethylene from the argon purge stream.
U.S. Pat. No. 5,233,060 discloses a direct-oxidation ethylene oxide process by reacting a feed gas stream including ethylene and a commercially-pure oxygen in one or more reactors and absorbing out ethylene oxide from the product stream from the one or more reactors in a first absorption zone. Unreacted ethylene is then removed from an ethylene-rich argon purge gas stream via an adsorber and a stripper in combination. Recovered ethylene is recycled to the feed gas stream. An ethylene-lean argon purge gas stream is removed.
U.S. Pat. No. 5,952,523 discloses a method for producing vinyl acetate using ethylene, acetic acid and argon containing oxygen that maximizes selectivity and minimizes ethylene loses to purge.
The prior art generally involves dedicated separation units for ethylene recovery. In many cases, ethylene is obtained at low pressure and a dedicated compressor must be used to recompress it to the operating pressure of the reactor. Due to incomplete recovery, a small volume of vent stream containing hydrocarbon is still generated. This vent gas must be either incinerated or burned as a fuel. Also, if the recovered ethylene contains impurities, complex controls will be required to prevent their build up in the recycle loop.
It is an object of the present invention to provide a process and apparatus for recycling ethylene from the purge stream of an alkene derivative process.
It is another object of the present invention to provide a process and apparatus for integrating an alkene derivative process with an ethylene process to effectively minimize alkene loss in the purge stream of the alkene derivative process.
It is yet another object of the present invention to provide a process and apparatus for recovering ethylene from the purge stream of an ethylene oxide process.
It is yet another object of the present invention to provide a process and apparatus for recovering the ethylene from the purge stream of a vinyl acetate monomer process.
This invention is related to a process for integrating an alkene derivative process with an ethylene process comprising the steps of (a) feeding a hydrocarbon feedstock to an alkene process to produce alkene; (b) reacting alkene with oxygen and reactants to produce an alkene derivative such as an ethylene derivative, a propylene derivative and other derivatives; (c) purifying the alkene derivative to produce a purified alkene derivative segment and an unconverted reactant-based segment; (d) recovering and removing the purified alkene derivative; (e) removing undesirable by-product gas from the unconverted reactant segment and feeding a first portion of the treated unconverted reactant segment to the reacting process in step (b) and then removing from the second portion of the treated unconverted reactant segment any components that are incompatible for use as feedstock in step (a); and (f) feeding the further treated second portion of the unconverted reactor segment of step (e) with the feedstock of step (a) into the alkene process.
This invention also relates to an apparatus for integrating an alkene derivative process with an alkene process comprising an alkene process having means for producing alkene from hydrocarbon feedstock and means for discharging the alkene to a reactor unit. The reactor unit has means for producing an alkene derivative from oxygen, alkene from an alkene source and reactants, and means for discharging the alkene derivative to a product recovery unit. The process gas treatment unit having means for removing undesirable gas from the unconverted reactants and means for discharging a portion of the undesirable gas-free unconverted reactants to the reactor section and the remainder to a purge gas treatment unit. The purge gas treatment unit has means for removing components that are incompatible for use in the alkene process and means for discharging the component-free unconverted reactants to the alkene process.
The current invention combines the alkene derivative process with the alkene process. The alkene derivative process purge stream is sent to the alkene process train. The alkene process train cracks hydrocarbons to produce alkenes (such as ethylene and propylene) for the alkene derivative processes. As will be discussed, the purge stream enters the train at one of several possible stages and travels with the hydrocarbon feed. Inerts from the purge stream are removed at a demethanization stage of the alkene process. The alkenes are then sent to the alkene derivative processes, while the separated inerts can then be sent off as usable fuel streams.